justthismorning's Communications in Binary is a wonderful Uni AU with Arthur and Merlin as room-mates. The character interplay is wonderful as Arthur, the OCD prat slowly unwinds thanks to Merlin the computer geek.
Then there is Camelot: A Romance With Rollercoasters by glitterdash, another modern AU set at the Camelot theme park. Beautifully witty with great characterisations, Arthur being the son of the business tycoon owner and Merlin being one of the employees who gets lumbered with Arthur doing "work experience".
And thirdly I read The Butcher of Camelot by dameange, a crossover of Merlin, Criminal Minds and Buffy. Sounds odd, but handled incredibly well so that having the BAU in Camelot was totally believable. It is not a forget the time zone type crossover, it is a time travel, blend the universes type and is done very well.
Now I have to decide what to read next ... decisions, decisions ...